Observation and Quadrupole Moment Measurement of the First Superdeformed Band in the A~60 Mass Region

Transcription

1 Observation and Quadrupole Moment Measurement of the First Superdeformed Band in the A~60 Mass Region Svensson, C. E.; Baktash, C.; Cameron, J. A.; Devlin, M.; Eberth, J.; Flibotte, S.; Haslip, D. S.; LaFosse, D. R.; Lee, I. Y.; Macchiavelli, A. O.; MacLeod, R. W.; Nieminen, J. M.; Paul, S. D.; Riedinger, L. L.; Rudolph, Dirk; Sarantites, D. G.; Thomas, H. G.; Waddington, J. C.; Weintraub, W.; Wilson, J. N.; Afanasjev, A. V.; Ragnarsson, Ingemar Published in: Physical Review Letters DOI: /PhysRevLett Link to publication Citation for published version (APA): Svensson, C. E., Baktash, C., Cameron, J. A., Devlin, M., Eberth, J., Flibotte, S.,... Ragnarsson, I. (1997). Observation and Quadrupole Moment Measurement of the First Superdeformed Band in the A~60 Mass Region. Physical Review Letters, 79(7), DOI: /PhysRevLett General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. L UNDUNI VERS I TY PO Box L und Download date: 27. Jan. 2019

2 Observation and Quadrupole-Moment Measurement of the First Superdeformed Band in the A 60 Mass Region C. E. Svensson, 1 C. Baktash, 2 J. A. Cameron, 1 M. Devlin, 3 J. Eberth, 4 S. Flibotte, 1 D. S. Haslip, 1 D. R. LaFosse, 3 I. Y. Lee, 5 A. O. Macchiavelli, 5 R. W. MacLeod, 5 J. M. Nieminen, 1 S. D. Paul, 2 L. L. Riedinger, 6 D. Rudolph, 7 D. G. Sarantites, 3 H. G. Thomas, 4 J. C. Waddington, 1 W. Weintraub, 6 J. N. Wilson, 1 A. V. Afanasjev, 8, * and I. Ragnarsson 8 1 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada, L8S 4M1 2 Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee Chemistry Department, Washington University, St. Louis, Missouri Institut für Kernphysik, Universität zu Köln, D Köln, Germany 5 Nuclear Science Division, Lawrence Berkeley Laboratory, Berkeley, California Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee Sektion Physik, Ludwig-Maximilians-Universität München, D Garching, Germany 8 Department of Mathematical Physics, Lund Institute of Technology, S Lund, Sweden (Received 6 May 1997) A high-spin rotational cascade of six g rays has been observed in 62 Zn. The quadrupole moment of e b measured for this band corresponds to a deformation b The properties of this band are in excellent agreement with calculations that predict high-spin superdeformed bands in 62 Zn with deformations b These results establish a new region of superdeformation for nuclei with neutron and proton numbers N, Z [S (97)03901-X] PACS numbers: Re, Ky, Lv, e In recent years the study of superdeformed (SD) bands has been one of the most exciting topics in nuclear structure research. Since the first observation of highspin superdeformation in 152 Dy [1], SD bands have been identified in many nuclei in the A 190, 150, 130, and 80 mass regions [2]. Based on calculations of large SD shell gaps in the single-particle energy levels for particle numbers N, Z [3,4], SD bands have also been predicted in the A 60 mass region [5]. Because of the limited number of particles in these nuclei and their proximity to the N Z line, superdeformation in this mass region is of particular interest. Studies of A 60 SD bands will enable comparisons between mean-field cranking models of SD nuclei and large-scale shell model calculations [6], studies of the onset of isospin T 0 neutron-proton pairing at high rotational frequency [7,8], and investigations of the loss of collectivity in these SD bands resulting from the limited spin content of their single-particle configurations [9]. However, because of a number of experimental difficulties, previous searches for these bands have been unsuccessful. In this Letter we report on the observation of a SD band in 62 Zn, the first observation of A 60 superdeformation. High-spin states in 62 Zn were populated via the 40 Ca 28 Si, a2p 62 Zn reaction at a bombarding energy of 125 MeV. A thin (500 mg cm 2 ) target enriched to 99.9% in 40 Ca was used, and the detection system consisted of the Gammasphere array [10] comprising 83 Compton-suppressed Ge detectors, 15 neutron detectors, and the Microball [11], a 4p charged-particle detector array consisting of 95 CsI(Tl) scintillators. The collimators were removed from the Ge detectors, allowing g-ray multiplicity and sum-energy information to be measured for each event [12]. With an event trigger requiring coincidences between at least three Ge detectors or between at least two Ge detectors and a neutron detector, events were recorded in five days of beam time. The a2p evaporation channel leading to 62 Zn comprised 10% of the total fusion cross section in this reaction and was selected by applying the total energy plane channel-selection method [13] to events in which an alpha particle and two protons were detected in the Microball. Events passing the selection criteria were unpacked into a symmetrized E g 2 E g correlation matrix which contained g-g coincidences, 95% of which were associated with 62 Zn. In addition to the normally deformed (ND) bands in 62 Zn identified in an earlier experiment [14], a new band consisting of a cascade of six g rays was observed. Figure 1 shows the g spectrum obtained by summing coincidence gates set on all of the members of this band. The g-ray energies, which have uncertainties of 62 kev, are labeled in this figure. Also labeled are the energies of known transitions in 62 Zn which firmly establish the new band as belonging to this nucleus. The observed intensity of the band in the plateau region was 1% of the channel intensity, as shown in the inset of Fig. 1(a). Although the band was weakly populated, single gates were sufficiently clean to confirm the mutual coincidence of all of the g rays except the 1993 kev transition. This transition was confirmed as a member of the band by summing double gates set on the band members in an E g 2 E g 2 E g coincidence cube. The stretched E2 character of the g rays was confirmed by their angular distributions (7) 1233(4)$ The American Physical Society 1233

3 FIG. 1. g-ray spectrum generated by summing coincidence gates set on all of the transitions in the new band (diamonds). (a) Energies label the band members and known transitions in 62 Zn; (b) expanded view of the band. The inset in (a) shows the intensity of the band relative to the channel. The inset in (b) shows spectra of the sum of the particle kinetic energies T p in the center of mass frame for the band (solid line) and the a2p channel (dashed line) obtained by triple gating on g rays from the band and from low-spin states in 62 Zn, respectively. Although a 3986 kev g ray was observed in coincidence with the band and appears to be a decay-out transition, the lack of statistics prevented a conclusive linking of this band into the 62 Zn decay scheme. The 3986 kev g ray carries only 8% of the intensity of the band and the remainder of the decay-out intensity is highly fragmented, as is typical of the decay out of SD bands in other mass regions (e.g., [15]). Although the band is not linked into the decay scheme, its high-spin nature is revealed by its decay into known high-spin states. The 1161 and 1791 kev g rays in Fig. 1(a), for example, arise from states with I p 14 1 and 13 2, respectively [14]. In addition, the excitation energy at which the band is fed can be accurately determined by measuring the sum of the particle kinetic energies T p in the center of mass frame [16]. The inset in Fig. 1(b) shows T p spectra in coincidence with the band (solid line) and low-spin states in 62 Zn (dashed line). The mean T p of 28.8 MeV for events feeding the band is 6.2 MeV below the average for the a2p channel. The corresponding mean total g-ray energy of 32.0 MeV for decays that pass through the band is thus 6.2 MeV larger than the 62 Zn average. These spectra show that the band is populated by only the highest excitation energy, and hence highest spin, components of 1234 the 62 Zn entry distribution. Based on the decay of the band into known high-spin states, a decay-out energy of at least 3986 kev, the excitation energy at which the band is fed, its intensity profile, and calculations (see below) which predict that the SD bands in 62 Zn become yrast for spins I $ 24 h, we estimate that the observed band extends from I 18 h to 30 h with a 62 huncertainty. In Fig. 2 we plot the dynamical moment of inertia J 2 as a function of rotational frequency for the new band and for typical SD bands from the other regions of superdeformation. In order to remove the mass dependence, all of the J 2 values have been divided by A 5 3 [17]. The scaled J 2 values for the 62 Zn band are comparable to those for SD bands in other mass regions, and we note that the 62 Zn band extends to the highest rotational frequency observed for a SD band. Although the large moment of inertia of the new band, its intensity profile, its feeding from the highest-spin components of the entry distribution, and its fragmented decay out into known high-spin states all support a SD interpretation, a definitive conclusion on the deformation of this band requires a measurement of its transition quadrupole moment Q t. Because of the high collectivity and high transition energies of this band, the state lifetimes (on the order of 1 fs) are much shorter than the time taken by the recoiling nucleus to leave the target (on the order of 100 fs). The entire band thus decays while the nucleus is slowing down in the target and Q t can be measured by the thin target Doppler shift attenuation method [18]. The measured fraction F of the full Doppler shift is plotted versus g-ray energy for a number of transitions in Fig. 3. Most of the transitions from states with I # 14 h decay outside of the target with constant shift F 0.872, the ND bands in 62 Zn decay on the same time scale as the time to recoil out of the target, and all of the transitions in the new band have very short lifetimes and are almost fully shifted. In order to extract the quadrupole moment of the new band, the slowing down of the recoiling nuclei in the target was modeled using the electronic stopping powers FIG. 2. Plot of the dynamical moment of inertia J 2 divided by A 5 3 versus rotational frequency for the new band in 62 Zn and for SD bands from other mass regions. For reference, the dashed line represents the moment of inertia of a rigid rotor with quadrupole deformation b

4 FIG. 3. Fractional Doppler shifts F for transitions from states with I # 14 h (diamonds), for a ND band in 62 Zn (squares), and for the new band (circles). The dashed lines at F 1.00 and represent the full shift and the constant shift for decays outside of the thin target, respectively. Calculated F curves for various Q t values are shown by solid lines. The new band is best fit by Q t e b. rations are calculated to be SD bands with deformations (b ), g 0 ± 15 ±. In all of the favored SD bands the neutron and proton orbitals up to the N, Z 30 SD shell gap corresponding to the f 7 2 -hole g 9 2 -particle configuration f g are filled. Different SD bands are formed depending on the orbitals occupied by the last two neutrons. The observed band in 62 Zn is assigned to one of the neutron SD configurations n f g 9 2 2,3. The Q t values calculated (as in [9]) for these bands in the spin range of interest, namely, Q t e b for the n f g configuration and Q t e b for the n f g configurations agree well with the measured value of e b. The n f g SD configuration shown in Fig. 4(c) is not a likely candidate for the observed band. The Q t of of Northcliffe and Schilling [19] scaled by the Ziegler and Chu [20] stopping powers for 4 He as suggested by Sie et al. [21]. The mean velocity of the recoiling nuclei was calculated as a function of time by integrating over the target and averaging over 100 reaction locations within the target. The decay of the nucleus was modeled by assuming a constant in-band Q t, and side feeding into each state was modeled by a single transition with the same lifetime as the precursor in-band state [22] and an intensity to match the observed intensity profile of the band. The fractional shifts calculated in this model for various Q t values are shown by solid curves in Fig. 3. The new band is best fit by Q t e b. Investigations of various decay models revealed that the uncertainty associated with the side feeding was insignificant in comparison with the uncertainty associated with the modeling of the slowing-down process which, when combined with the above statistical uncertainty, yields a Q t value of e b for the band. Assuming an axially symmetric shape, the corresponding quadrupole deformation is b We have carried out a theoretical analysis of highspin states in 62 Zn employing the configuration-dependent shell-correction approach with the cranked Nilsson potential [23,24]. Representative potential energy surfaces calculated for I p 20 2 and 24 2 are shown in Figs. 4(a) and 4( b), respectively, and the energies of favored highspin collective configurations are plotted versus spin in Fig. 4(c). At moderate spins the yrast configurations are triaxial signature-degenerate ND bands (b 2 0.3, g 30 ± ) which approach the g 60 ± noncollective axis at higher spins. These correspond to the smoothly terminating strongly coupled bands previously observed [14] in 62 Zn. For higher spins (I $ 24 h) the yrast configu- FIG. 4. Calculated high-spin properties of 62 Zn. Potential energy surfaces are shown for (a) I p 20 2 and (b) I p Equipotential lines separated by 0.25 MeV are shown up to 5.5 MeV above the minima. (c) Energies of favored collective configurations relative to an I I 1 1 reference plotted versus spin. The SD configurations are labeled by their neutron and proton f 7 2 holes and g 9 2 particles and by their equilibrium deformations at I 22 h. Solid (dashed) lines represent positive (negative) parity, and closed (open) symbols denote signature a 0 (a 11). Terminating ND states are encircled. For I. 30 h the yrast lines for different combinations of parity and signature are shown by lines of different type without symbols. The states corresponding to the maximum spin extracted from the low-spin SD configurations are indicated by large open squares. Note that these states are collective and that the SD bands can be extended to higher spins, as indicated by the dotted lines. 1235

5 e b calculated for this band is outside of the experimental uncertainty on the measured value, and the absence of the signature-partner band that should be observed with a single signature-degenerate f 7 2 hole also argues strongly against this configuration. It is expected that future experiments in this mass region will establish links between SD bands and ND states. The definite configuration assignments resulting from such links will provide important information on the, as yet unknown, single-particle energies at large deformation in this mass region. We also note that the states corresponding to the maximum spin extracted from the low-spin SD configurations [large open squares in Fig. 4(c)] occur at spins only slightly higher than observed in this experiment. Although these states are collective and the SD bands do not terminate in the usual sense, their properties in the I 30 h 40 h range are strongly affected by the limited spin content of their single-particle configurations. In summary, we have observed a cascade of six g rays in 62 Zn that forms a rotational band over an estimated spin range of I 18 h 30 h. The scaled J 2 moment of inertia of this band is comparable to those of SD bands in other mass regions, and the measured quadrupole deformation of b is in excellent agreement with theoretical calculations which indicate that SD bands with b become yrast in 62 Zn for spins I $ 24 h. These results establish a new region of superdeformation in nuclei with neutron and proton numbers N, Z A systematic study of SD bands in this mass region is clearly required to define the limits of this newest region of superdeformation and to extract detailed information about the single-particle energies at large deformation for these light nuclei. This work has been partially funded by the Natural Sciences and Engineering Research Council of Canada, the U.S. D.O.E. under Contracts No. DE-AC05-96OR22464 and No. DE-AC03-76SF00098 and Grants No. DE-FG05-88ER40406 and No. DE-FG05-93ER40770, the German BMBF under Contracts No. 06-OK-668 and No. 06-LM- 868, the Swedish Natural Science Research Council, and the Royal Swedish Academy of Sciences. *Permanent address: Nuclear Research Center, Latvian Academy of Sciences, LV-2169 Salaspils, Latvia. [1] P. J. Twin et al., Phys. Rev. Lett. 57, 811 (1986). [2] X.-L. Han and C.-L. Wu, At. Data Nucl. Data Tables 63, 117 (1996). [3] R. K. Sheline, I. Ragnarsson, and S. G. Nilsson, Phys. Lett. 41B, 115 (1972). [4] J. Dudek et al., Phys. Rev. Lett. 59, 1405 (1987). [5] I. Ragnarsson, in Proceedings of the Workshop on the Science of Intense Radioactive Ion Beams, edited by J. B. McClelland and D. J. Vieira (Los Alamos National Laboratory Report No. LA C, 1990), p [6] D. J. Dean, in Proceedings of the Conference on Nuclear Structure at the Limits (Argonne National Laboratory Report No. ANL PHY-97 1, 1997), p [7] K. Nichols and R. A. Sorensen, Nucl. Phys. A309, 45 (1978). [8] E. M. Müller et al., Nucl. Phys. A383, 233 (1982). [9] A. V. Afanasjev and I. Ragnarsson, Nucl. Phys. A608, 176 (1996). [10] I.-Y. Lee, Nucl. Phys. A520, 641c (1990). [11] D. G. Sarantites et al., Nucl. Instrum. Methods Phys. Res., Sect. A 381, 418 (1996). [12] M. Devlin et al., Nucl. Instrum. Methods Phys. Res., Sect. A 383, 506 (1996). [13] C. E. Svensson et al., in Proceedings of the Conference on Nuclear Structure at the Limits (Argonne National Laboratory Report No. ANL PHY-97 1, 1997), p. 209; Nucl. Instrum. Methods Phys. Res., Sect. A (to be published). [14] C. E. Svensson et al. (to be published). [15] A. Lopez-Martens et al., Phys. Rev. Lett. 77, 1707 (1996). [16] D. P. Balamuth, T. Chapuran, and J. W. Arrison, Nucl. Instrum. Methods Phys. Res., Sect. A 275, 315 (1989). [17] D. F. Winchell et al., Phys. Lett. B 289, 267 (1992). [18] B. Cederwall et al., Nucl. Instrum. Methods Phys. Res., Sect. A 354, 591 (1995). [19] L. C. Northcliffe and R. F. Schilling, Nucl. Data Tables A 7, 233 (1970). [20] J. F. Ziegler and W. K. Chu, At. Data Nucl. Data Tables 13, 463 (1974). [21] S. H. Sie et al., Nucl. Phys. A291, 443 (1977). [22] H. Savajols et al., Phys. Rev. Lett. 76, 4480 (1996). [23] T. Bengtsson and I. Ragnarsson, Nucl. Phys. A436, 14 (1985). [24] A. V. Afanasjev and I. Ragnarsson, Nucl. Phys. A591, 387 (1995). 1236